1. Field of the Invention
This invention relates to polarization-transforming fiber, specialized fiber, polarization optics in specialized fiber, optical fiber circuitry and networks involving use of circular light, all-fiber optical architecture for gyroscope, electric-current sensor, and coherence optical fiber communication, fiber-optic interferometers, Sagnac interferometer, etc. In particular, this invention relates to broad-band fiber-optic wave plates.
2. Description of the Related Art
In any optics laboratory, conventional bulk-optic wave plates are versatile elements widely used in lightwave experiment. Thus, a bulk-optic quarter wave plate is commonly used to convert a linear light into a circular light, or conversely a circular light into a linear light. A bulk-optic half wave plate is commonly used to change the orientation of a linear light. In modern fiber-optic systems, while such bulk-optic wave plates can be incorporated with the optical fiber circuitry to perform a prescribed state-of-polarization (SOP) transform function, the overall composite layout is likely bulky and vibration-sensitive, requiring careful and deliberate manual tuning and adjustments in maintenance of the system operation. More remarkably, a bulk-optic wave plate structured by a calcite crystal of precise thickness is inherently narrow-band, and this is oftentimes a restrictive feature for its inclusion in modern optical fiber communication and sensor technology. As well-known, the narrow-band inherence of a conventional bulk-optic wave plate is due to the fact that this optical element made by a calcite crystal works on the principle that the o-wave and the e-wave (o, e, short for ordinary and extraordinary) acquire a precisely specified phase difference in passing the element, and this incurs the extremely wavelength-sensitive characteristics of the element.
In the realm of fiber optics, a tiny length of hi-bi (highly birefringent) optical fiber is capable, in principle, of performing an SOP transform if the incident linear light is intentionally to be aligned off-axis, preferably at a 45° angle from either principal axis of the hi-bi fiber. The two othorgonal components, or polarization modes, will beat along the fiber axis, so that the SOP of light will undergo all changes in half a beat-length. In actual practice, however, it scarcely appears practical to use a short hi-bi fiber section of very precise length to serve the purpose of a desired SOP transformation, particularly in view of an all-fiber optical circuitry wherein each fiber element is preferably in-line spliced at either end of the element. Like the case of bulk-optic wave plates, the SOP transform based on beating of polarization modes in hi-bi fiber is also inherently narrow-band, extremely sensitive to the operating wavelength and the length of the tiny fiber section.
It is on said background that this invention is brought into being in the spirit of the multifold advantageous features of the invention, e.g., the invented fiber-optic element is capable of performing a multitude of SOP transforms almost effortlessly without the need of tedious adjustment and aligning, the convenience for in-line use in an all-fiber optical network in which the invented fiber element can be spliced anywhere, and most importantly, the broad-band inherence of the invention whose working mechanism is totally novel as to be described in more detail subsequently.